The present invention provides a cell for use in a one-step cell-based assay for an extracellular ligand (e.g., IFNα) that initiates a ligand-specific signal at the nucleus of the cell and for neutralizing antibodies against the extracellular ligand. The cell-based one-step assay allows both the extracellular ligand concentration and the neutralizing antibody titer to be quantified in a single sample (e.g., serum) without the need for sample dilution and addition of exogenous extracellular ligand.